What do IBU and "Flavor Intensity" mean?

Here at Hopsy, we're all about beer. Our minikeg labels include two statistics you may not have seen before.

IBU: IBU is the abbreviation for "International Bitterness Units" scale. This number is a gauge of a beer's bitterness, measured in parts per million of isohumulone, an acid from hops, found in the beer. Isohumulone gives beer a bitter bite. Hoppy beers, like IPAs, have higher IBUs, while beers with fewer hops, like a wheat beer, have lower IBUs.